As devices formed on a substrate, such as a semiconductor wafer, have been becoming finer in recent years, a solder bump, which is a protrusion-shaped metal electrode, is frequently used. FIG. 14 shows an example of a cross-section of a multilayer structure for forming the solder bump. As shown in FIG. 14, a dielectric layer 2 is formed on a silicon substrate 1, and a barrier metal layer 3 and a copper seed layer 4 are formed in this order on the dielectric layer 2. A copper bump layer 5 is then formed on the copper seed layer 4, and a barrier layer 8 is formed between the copper bump layer 5 and a solder layer 7. This barrier layer 8 is a layer for preventing an interdiffusion between the solder layer 7 and the copper bump layer 5. Nickel (Ni), palladium (Pd) or the like is typically used for the barrier layer 8. A lead-free solder, such as tin (Sn), tin-silver alloy (Sn—Ag alloy), or tin-copper alloy (Sn—Cu alloy), is used for the solder layer 7.
In such a multilayer structure, it is necessary to etch and remove the copper seed layer 4 in locations where the copper bump layer 5 is not formed. Typically, wet etching using an etching liquid (i.e., a chemical liquid) or dry etching utilizing plasma is used for etching of a metal film. The wet etching is used for etching the copper seed layer 4, because a wet etching apparatus is relatively simple in its structure and low-cost. An SPM liquid (a mixture of sulfuric acid and hydrogen peroxide), ferric chloride, or the like is used as a conventional etching liquid in this wet etching. By using such an etching liquid, only the copper seed layer 4 can be etched away as shown in FIG. 15.
As described above, a metal used for the barrier layer 8 is typically nickel or palladium. It has recently been proposed to substitute cobalt (Co) for nickel or palladium from a viewpoint of safety and costs. However, if the conventional etching liquid described above is used in a case where cobalt is used as the barrier layer 8, cobalt dissolves more quickly than copper due to a local cell action caused by a contact between dissimilar metals. More specifically, the barrier layer 8 dissolves before the copper seed layer 4 dissolves, and as a result, the copper seed layer 4 hardly dissolves. Moreover, since the barrier layer 8 retreats, a sufficient bonding strength between the solder layer 7 and the barrier layer 8 cannot be obtained.
International patent publication No. WO 2011/074589 discloses an etching liquid which includes hydrogen peroxide and citric acid and can selectively etch copper without etching nickel. However, if the etching liquid disclosed in this International patent publication is applied to the above-described multilayer structure including the copper layer and the cobalt layer, the cobalt layer dissolves while the copper layer hardly dissolves.